1. Field of the Invention
The present invention relates to a position detector that converts, into position information, output signals from a position sensor which outputs two signals that sinusoidally vary at a pitch of wavelength λ with respect to a measured displacement and have phases shifted from each other by 90 degrees.
2. Description of the Related Art
At a rotation shaft of a rotary table or the like of a machining tool, a drive scheme achieved by combining a reduction gear such as a worm gear with a servo motor was previously employed. However, drive schemes using reduction gears are disadvantageous in that accuracy degradation occurs due to backlash, and increase of rotational speed is limited. Accordingly, in recent years, a direct motor drive scheme achieved by incorporating a built-in motor to facilitate accomplishing high accuracy and high speed has been employed at the rotation shaft.
In a rotary table or the like of a machining tool, high-accuracy position detectors are conventionally employed for providing feedback in performing position control. In order to increase the position detection accuracy, these position detectors are configured using a position sensor which outputs two signals that sinusoidally vary at a small pitch with respect to a measured displacement and have phases shifted from each other by 90 degrees. Such a position sensor is used because, even when the resolution accuracy (hereinafter referred to as interpolation accuracy) within the pitch obtained by performing interpolation processing with respect to the two signals is poor, as long as the pitch is small, the ratio by which the interpolation accuracy influences the actual position detection accuracy remains small.
However, in this type of position detector, when rotation about the axis is performed at a high speed, the frequency of the output signals becomes excessively high. For this reason, a position detector of this type that can be used at high rotational speeds has not been available. Accordingly, the rotation shafts driven by the direct motor drive scheme which facilitates high-speed rotation were subjected to restraints in high-speed performance by the position detectors available.
In light of this background, there has been a need for position detectors capable of enhancing the interpolation accuracy even when a position sensor that outputs signals having long pitches in response to a measured displacement is used. Position detectors of this type are disclosed in JP 4-136715 A, JP 2003-14440 A, JP 2005-156348 A, and the like.